The present invention relates to belt presses for pressing composite assemblies and is more particularly an improvement on the technologies disclosed in U.S. Pat. No. 4,517,148. This and each of the other patents mentioned in this disclosure are hereby incorporated by reference in their entireties.
Structural wood products can be manufactured from long relatively thin strands of wood by coating the strands with an adhesive, arranging the strands side-by-side in a lengthwise dimension of the lumber product and subjecting the arranged strands to heat and compression. A high strength dimensional wood product is thereby formed, and this process is disclosed for example in U.S. Pat. No. 4,061,819. Belt presses are typically used in processes for the manufacture of composite wood products, and examples thereof are shown in U.S. Pat. Nos. 3,120,862; 3,723,230; 3,792,953; 3,851,685; 3,993,426; 4,043,732; 4,850,846; and 4,410,474. The belt presses include facing endless belts between which the material is compressed and platens and anti-friction devices which hold the belts in pressure engagement with the materials conveyed therebetween. The inlet end of the press belts and the platens over which they run converge towards one another to form a compressing zone. As the strands enter the compressing zone they are generally free to move with respect to one another. However, as the belts converge within this zone the strands have their positions set, in a "lock-up" position, with respect to one another. After lock-up further compression of the material results from the further convergence of the press belts. Since lock-up usually occurs in a curved area, the material is curved as it is being pressed. After the compressing zone the lock-up material is passed through a parallel belt zone.
Although the resulting material passing out of the parallel belt zone is planar, the curvature of the strands at lock-up is "remembered" as an internal stress therein. Where the end products are thin and planar these internal stresses do not present a problem. For relatively thick products, however, such as dimensioned lumber, these internal stresses present a problem when the thick product is cut longitudinally. When so cut the internal stresses are released and the two resulting halves bow in opposite directions. In other words, with single radius infeeds which are tangent to the parallel belt of the press prestresses in the wood, which are a function of the infeed radius, occur and when later resawn axially the prestresses are released and the final product curves. This problem has been addressed in the past by using a very large compression radius.
Increasing the entire infeed radius may not always be possible, however, as the large radius may not provide a large enough infeed opening for the projected point of lock-up. Additionally, the radius at times cannot be extended sufficiently to open the press opening where there are physical limitations on the length of the press bed. Also, a long press has been needed to obtain an opening large enough to "bite" on thick incoming mats. For example, when a twelve inch product is being compresssed, a twenty-four to thirty-six inch opening is initially required. It thus takes a very long press bed in proportion to the extra press depth to gain a smooth even compression.
In one known press the required shape of the compression radius and the parallel bed was cut from a steel plate about one-and-a-half inches thick. Four shaped "ribs" were cut out of an about ten inch deep, one and a half inch wide and fifteen foot long plate in about thirty inch wide sections and were bent over the plate. The ribs acted as stiffeners to hold the shape of the plate. The assembly was welded together, and two such platen assemblies were mounted facing each other. Roller chains covered the platens and moving bearing plates covered the rollers. A tensioned and driven belt provided the traction force and faced directly on the product. This is a constant radius type of fabricated radiused platen.
An example of another known press, a stepped radius press, is the commercial press available from Eduard Kuesters of Krefred, West Germany. It uses a flat platen bent at specific discrete points defined by machined slots in the back thereof. Each flat section, which is typically eighteen inches long, is supported with a wedge block and frame. In this press, roller chain assemblies separate the steel belt which is driven by drums from the platen and form the friction reducing medium. The effective radius of the platen area, which can be defined by the intersection of the perpendicular bisectors of adjacent platen segments, is for example for a three-quarters of a degree bend between two sections or segments one hundred and twelve feet. The prestressing can be characterized by the amount of bow in beams cut from the product. A forty-eight foot long beam produced by this press and cut from the face of the billet typically has an often unacceptably large bow of two-and-a-half to four inches.